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Research in the Sessa lab focuses broadly on plant systematics and understanding the evolutionary and ecological processes that shape plant diversity. Lab members work on a number of projects around this central theme, with phylogenetics as the unifying conceptual framework. Phylogenies frame our research in a number of related areas, including: basic understanding of relationships among taxa, reticulate evolution, diversification rates and biogeographic history of organisms, community phylogenetics, and trait evolution using comparative methods.

Organismally, we have a strong focus on the seed-free vascular plants: ferns and lycophytes. The primary question motivating much of our research is:

What ecological and evolutionary processes have generated, and maintain, fern and lycophyte diversity on earth?

Ferns are the second largest group of vascular land plants (ca. 10,000 species), and lycophytes (ca. 1,300 species) are sister to ferns+seed plants. Ferns and lycophytes have historically been underrepresented compared to other plant lineages in studies of phylogenetics, genomics, and physiological ecology, and as a result many questions remain unanswered about their phylogenetic relationships, genome structure, and the physiological traits that have allowed them to persist and diversify. In particular, polyploidy and other reticulate processes (e.g., hybridization, introgression) are recognized as critical phenomena that have shaped the evolutionary trajectories of many plant lineages, and ferns have the highest rates of polyploidy among vascular plants. Research on ferns and lycophytes in our lab focuses on several major areas of inquiry, including biogeographic patterns, diversification processes, mating systems, community assembly, genomics/transcriptomics, and physiological ecology, all of which we generally approach from the context of phylogeny.

Our work, whether on ferns and lycophytes or other systems, depends on data collected via field and experimental studies and Sanger and next-generation sequencing. We integrate techniques from several disciplines, including computational and evolutionary genomics, phylogenetic systematics, physiological ecology, and traditional specimen-based botany. There are many ongoing projects in the lab, including those listed below.